1. Field of the Invention
This invention relates to improvements in methods, circuits, and apparatuses for optically detecting the presence of a flowable substance, such as ink ejected from a thermal print head or the like, and, more particularly, to such improvements and methods that do not require absolute light levels to enable detection of such substances.
2. Relevant Background
It is often desired to detect the presence of a flowable substance in a particular region, for instance, in the operation of thermal ink ejecting print heads that may be associated, for example, with printers for use in conjunction with computers or other systems. Typically, such thermal ink ejecting print heads have a plurality of apertures from which the ink is ejected after being heated by one or more heating elements within the print head. One mechanism that is frequently employed uses a heater to which ink is delivered by capillary action of an ink delivery tube. As an ink drop comes into proximity with the heater, the ink is partially vaporized into an expanding steam component that explosively expels the remaining portion of the ink drop from the aperture of the print head. The capillary action of the delivery tube brings additional volumes of ink to form additional ink drops for successive expulsion from the aperture of the print head. Thus, although the "jet" of ink being expelled from the print head appears to be a continuous stream of fluid, in reality, the stream is a plurality of successive explosively ejected ink drops from the aperture that is associated with the heating element as ink is delivered drop-by-drop from the capillary tube.
The frequency at which the drops are produced is dependent upon a number of factors, including the rate at which the capillary action can deliver ink to the heater, the rate at which the heater can be energized, the physical dimensions of the apertures and chambers in which the ink is partially converted to steam, and the like. In a typical system, for example, the ink drop ejection rate is between about 3-5 kHz.
In the heating mechanism associated with the ink drop expulsion, if the power to the heating element is insufficient to produce the steam pressure necessary for the expulsion of the ink drop, no ink will be expelled from the print head. It has been found, in fact, that the power threshold for enabling ink ejection is quite sharp. On the other hand, if an excessive amount of power is applied to the heater, portion of the ink drop may be ashed. This unnecessary ashing may deposit on the heater and other print head parts, ultimately resulting in degraded operation, and in many cases, complete failure of the print head.
At various times, for example upon completion of the manufacture of the printer in which the print head is included, or, ideally, upon individual power up events in which the printer is used, the power level applied to the heaters of the print head may be adjusted to insure that it is above the ink producing threshold, but below the level that produces undesired ink disintegration. Since many manufacturing tolerances and part variations exist, the optimum power ranges may vary significantly from printer to printer. However, heretofore, the methods and circuitry that have been proposed to assure proper power level determination are relatively complicated and expensive to implement.
For example, one of the problems that exists in determining whether the power level applied to the heater is above the threshold required to produce ink ejection is in the detection of the ink that actually has been ejected from the print head. More specifically, the ink that is ejected does not necessarily completely block an optical path that otherwise could be monitored with inexpensive optical sensors or the like. The ink volume, in fact, may result only in a partial obstruction of the optical path, resulting in risk of non-detection of the presence of the ink and overpowering of the heaters.